As you can see in a common graph taken from  and representing the impulse current from a lightning stroke below, the notation 1.2/50 µs means that the raising step lasts for 1.2 µs, while the falling step is always slower and lasts (50-1.2)=48.8 µs. 50 µs is the total duration of the impulse.
 Deepak Subedi, Matti Lehtonen. (2017). Lightning Induced Over-voltages in Power Transformer and Voltage Spikes in Connected Load